The present invention relates generally to phase detectors, and more particularly to a phase detector of the type which employs sample and hold techniques.
Phase detectors are commonly used in a wide variety of applications. In most applications, the phase detector is a critical component which in large part determines the operational characteristics of the entire system. One of the most critical of these applications involves the use of phase detectors in frequency synthesizers. Frequency synthesizers generally employ voltage controlled oscillators (VCO's) whose frequency of oscillation may vary over a wide range of frequencies, dependent upon a control voltage which is supplied to the frequency control input thereto. Since these voltage controlled oscillators tend to be rather unstable, it has been the practice to include an extremely precise reference oscillator to produce a reference signal. The output of the VCO is compared to the reference signal by means of a phase detector. If the phase detector itself generates reasonably large noise components, then the benefits derived from the inclusion of a reference oscillator are in some measure lost. Consequently, it is highly important that the phase detector included in a system of this sort be as entirely noise-free as possible.
One method of achieving this phase detection is disclosed in a patent to Brase et al., U.S. Pat. No. 3,337,814. The method disclosed in this patent involves the use of a flip-flop as the phase detection element. The flip-flop is triggered from one state to another by the two signals which are being phase compared so that the duty cycle of the output of the flip-flop is dependent upon the phase difference between the two input signals. Unfortunately, this frequency control scheme requires a large amount of filtering, and has a relatively low loop gain.
A more promising method of phase comparison is disclosed in the patent to Blachowicz et al., U.S. Pat. No. 3,495,096. In this patent, one of the signals which is to be compared with the phase of the other signal is converted into a waveform which is of generally triangular shape. A sample and hold circuit then periodically samples this waveform at points in time determined by the second signal. Since each of the samples is taken during a finite interval, the amplitude of the triangular waveform would change during the sampling process. In order to prevent the changing amplitude of the triangular waveform from adding a spurious component to the output of the sample and hold device, means are provided for preventing the triangular waveform from changing amplitude during the sampling process. The net result is a phase detection scheme which has a relatively high loop gain, and which provides a quite stable DC output level. Little filtering is thus required.
Since any noise component present at the output of the phase detector will be "amplified" by the voltage controlled oscillator, it would be desirable to reduce the noise component even below the level provided by the Blachowicz system.